The way in which enkephalins and narcotic analgesics act upon single neurons is investigated in rodent neural tissue during the development of physical dependence and tolerance to narcotics. The hypothesis that the primary receptor mediated action of narcotics is to stabilize excitable membranes is tested by intracellular and extracellular recording techniques, iontophoretic application of narcotic agonists or antagonists, manipulation of the ionic environment, inhibition of ion transport, or interference with cyclic adenosine monophosphate metabolism.